The invention relates to active matrix devices, such as an active matrix LCD display. In particular the invention relates to driving the bias inversion circuitry of active matrix devices.
Electronic devices, such as notebook computers and personal data assistants, to name a couple, have liquid crystal displays (LCDs) for presenting information to users of the electronic devices. In comparison with conventional video graphic cathode-ray tube (CRT) terminals used with desktop computers and televisions, LCDs consume less power, are thinner, lighter in weight, and typically are more expensive. While LCDs have some visual display quality benefits over CRTs, such as exceptional geometric linearity and sharpness, LCDs have typically lagged CRTs in other visual display quality areas such as viewing angle, brightness, and display speed.
An LCD is typically formed of an array or matrix of individual pixels that determine the LCD resolution. The visual display quality properties are generally determined by how a pixel is fabricated and driven on an LCD. A pixel is the smallest element of a display surface that can reproduce the full range of luminance and colors of the LCD. For color displays, each pixel can further be broken up into three sub-pixels that represent the red, green, and blue colors used generate the overall perceived color of the pixel. Thus, a sub-pixel is the smallest driven element in an AM-LCD. For a monochrome display, a single sub-pixel may represent an entire pixel. When a pixel is described as being white, it means that each sub-pixel element for the pixel is being driven to its maximum luminance. When a pixel is described as black, it means each sub-pixel is set at it minimum luminance.
Manufacturers of LCDs have developed new LCD technology known as active-matrix (AM) LCD. The active-matrix LCD incorporates additional components on the display to drive each sub-pixel such that the viewing angle, brightness (perceived luminance), and display speed is improved to levels which allow AM-LCDs to compete with CRTs. This competition exists not only for portable electronic devices but also as display monitors for desktop computers, televisions, and projectors, to name a few.
However, one visual display quality that persists for AM-LCDs is display xe2x80x9cflickerxe2x80x9d. Flicker is an intermittent change in light intensity perceived by an eye. Flicker is caused by the manner in which the sub-pixels on the display are driven by circuitry that is used to remove direct-current bias voltage to the sub-pixel. If the direct-current bias voltage is not removed, permanent physical display artifacts may form on the display surface irreparably damaging the display. Manufacturers of AM-LCDs have tried several different approaches to reducing display flicker. Unfortunately, these different approaches still produce flicker when commonly used patterns are displayed on the AM-LCD screen. Since the flicker occurs when certain common patterns are displayed, the user is often annoyed and will at times call the manufacturer of the electronic device to inquire if their device is defective. Since the flicker goes away when the pattern on the AM-LCD screen changes, sometimes the user believes that their device is malfunctioning intermittently and may try to return it. Manufacturers of electronic devices are unable to help the user of the device except to try to explain the flicker characteristic. This type of response by a manufacturer may lead to frustration by the user since the user""s problem is not being corrected but just explained away. Accordingly, as the cost of AM-LCDs approach that of CRTs and become more popular, user dissatisfaction, the number of service calls and product returns will increase if a solution to the display flicker quality issue is not found.
Another problem with flicker, potentially serious, is that display flicker can cause discomfort in varying degrees depending on the individual, and can precipitate epileptic seizures in susceptible individuals (see Flat Panel Display Measurements Standard Version 1.0, Video Electronics Standards Association, Ver. 1.0, Jun. 9, 1998, p. 92). The problem tends to be worse for frequencies near 10 Hz. Thus if any display has a substantial flicker component near 10 Hz, this is a cause for concern. Therefore, there is a need for AM-LCDs to reduce direct-current bias voltage in a manner that display flicker is reduced or substantially eliminated, in particular in the frequencies near 10 Hz.
An active matrix device has a plurality of drive signals, a plurality of select signals and an array of sub-pixels. Each of the sub-pixels has an electronic element connected to one of the drive signals and one of the select signals to display. The active matrix device also includes inversion circuitry coupled to the drive signals that has at least one Cole sequence generator. A Cole sequence generator provides a random, semi-random or pseudo-random sequence pattern. The inversion circuitry is capable of reducing the direct current bias voltage applied by the electronic element to the sub-pixel. The inversion circuitry is further capable of reducing flicker of the active matrix device.